Application Note 67
14 REV. 1.0 10/25/2000
COMPONENT SELECTION
CAPACITORS
Since the ML4423 uses an external R/C to set up the low
frequency oscillator, drift over temperature is directly
related to the component tolerances at this node. If tight
frequency tolerance is necessary this should be reflected
in the component tolerances for the anticipated operating
temperature range.
RESISTORS & CURRENT SENSE RESISTORS
For best results, use 1% resistors in the voltage feedback
path. Wirewound resistors must not be used in the current
feedback path. A wirewound resistor can add 200nH of
circuit loop inductance, or the equivalent of a 20” long
trace. With a 200ns fall time, 5A of current translates
into a 5V spike across the current sense resistor for that
period of time. This will trip the current limit on the
ML4423 and cause problems with the bootstrap circuit
and the associated level shifter.
LEVEL SHIFTERS
The ML4423 and the level shifter used to drive the upper
IGBTs must provide the correct polarity signal to the IGBT
or a shootthrough short circuit will occur. Some level
shifters do not incorporate a pulse filter which rejects
small pulse widths. This may cause the level shifter to
latch in one mode and not reset. This typically occurs
near the crest of the output phase waveform, so when the
latch is not reset the ML4423 feedback error increases
and you lose part of the output cycle. If the bootstrap cap
discharges to the undervoltage lockout of the level shifter
the output cycle may also drop out.
BOOTSTRAP DIODES
The diodes used for generating the bootstrap voltage must
be ultra-fast (< 75ns). Slow devices will have a long
recovery time and can cause dV/dt induced noise
coupling problems. The diode should have at least 100V
of margin over the maximum bus voltage.
POWER DEVICES
IGBTs are suited to compete with MOSFETs to 20kHz.
The co-pack IGBT’s have softer and faster internal diodes
than MOSFETs. The relatively slow and snappy
characteristics of MOSFET diodes (in freewheel mode,
common for motor control) may cause chaos on ground
referenced current limit schemes, and will require larger
R/C filtering on the ISENSE pin. EMI considerations will
also play a role in device selection.
POTENTIAL DESIGN ISSUES
Other design issues to consider include: power switch
size, power dissipation and heatsinking, gate drive power,
gate charge, cost and reliability, and PCB layout.
Figure 22f. Bus Power PCB Top Silkscreen
Figure 22g. Bus Power PCB Top Layer
Figure 22h. Bus Pow er PCB Bottom Layer
Figure 22i. Bus Power PCB Bottom Silkscreen